Generally, when a call signal is received from another person, a mobile communication device outputs via a speaker a ring tone or a melody to notify the user of the reception of the call signal.
Furthermore, as various functions, e.g., MP3 playback, digital multimedia broadcasting (DMB) service, games, etc., are embedded in mobile communication devices, the performance of speakers of mobile communication devices has become very important.
A speaker is a device for converting electric energy to mechanical energy by using a voice coil placed within a gap. The conversion takes place according to the Fleming's left hand rule, which states that, if a conductor via which a current is flowing is in a magnetic field, a force is applied to the conductor. In other words, a current signal of various frequencies is applied to a voice coil, the voice coil generates mechanical energy according to the intensity and frequency of the current and makes a diaphragm attached to the voice coil oscillate, and thus, a sound pressure of a predetermined magnitude audible to the human ear is generated. A device which generates a relatively low sound pressure and is used close to the human ear is generally referred to as a receiver, whereas a device which generates a relatively high sound pressure and is used a predetermined distance away from the human ear is referred to as a speaker.
Speakers may be categorized according to their structures into, for example, a cone type, a flat-panel type in which a voice coil is directly attached to a diaphragm, a dome type which uniformly spreads reproduced sound, a hone type which is for a megaphone and has high directivity, a ribbon type which precisely reproduces sound characteristics, an electrostatic type (condenser type) which outputs fine sound and has a relatively small size, etc. Furthermore, according to sound quality, speakers may be categorized into a woofer, a tweeter, and a mid-range unit.
An electrostatic speaker includes an audio signal electrode plate and a diaphragm. A high voltage is applied to the diaphragm, which is formed of a material with high surface resistance, and thus, the diaphragm can store positive or negative charges. An electrostatic attraction takes place between positive and negative charges between a stator, which is an audio signal electrode, and the diaphragm. An audio signal electrode plate, which is continuously changed according to audio signals, pushes the diaphragm at one side and pulls the diaphragm at another side, and thus, the diaphragm oscillates. The oscillation is reproduced as sound. An audio signal is converted into high pressure by an audio transformer, which is then applied to an audio signal electrode plate. The closer a distance between the audio signal electrode and the diaphragm is or the larger a voltage difference is, a greater force is generated, but there is a restriction. The restriction is a natural discharging phenomenon due to a high voltage difference. Therefore, it is impossible to infinitely increase a voltage difference or to arrange the audio signal electrode and the diaphragm too close to each other. Furthermore, it is necessary to consider a sufficient distance between the audio signal electrode and the diaphragm to reproduce the entire audible frequency band.
In a conventional electrostatic speaker, a single electrode plate is arranged, and a diaphragm has a single structure. Therefore, the conventional electrostatic speaker has relatively low sensitivity and low charging density. Furthermore, since a separate external power supply for a diaphragm is required, there are limits in designing an electrostatic speaker.